Add versioned docs with pydata or napari sphinx themes

[jni]

The PyData sphinx theme recently (0.8.0, Jan 2022) added a version switcher dropdown menu, documented here:

https://pydata-sphinx-theme.readthedocs.io/en/latest/user_guide/configuring.html#add-a-dropdown-to-switch-between-docs-versions

I'm still not clear on how to change docs deployment so that versions are uploaded to the right URL on gh-pages, in order to work with the dropdown. Inspiration might be found at scipy/scipy.

It occurs to me that the same infrastructure could potentially be used to deploy PR docs as well as stable ones...

CC @lucyleeow

[jni]

Closed by #173 #177 #178